Color filter arrays are employed in combination with sensors to define color images or in combination with display devices to permit color images to be viewed. A common approach to producing color filter arrays has been to use organic dyes embedded in a layer which has been patterned by various techniques to render the appropriate filter pattern. This approach has two significant disadvantages. The spectral characteristics of the filter are controlled by the absorbance curves of the dye and layer materials. Altering the spectral characteristics, therefore, requires altering the dye or layer material, which can be a difficult and time consuming process. Furthermore, the dyes may be subject to fading with time especially under harsh environmental operating conditions such as high light.
An alternative, which overcomes the disadvantages of the organic dye approach, has been to produce color filter arrays from interference filters made up of alternating layers of two dielectric materials with different refractive indices. Various combinations of pairs of dielectric materials, deposition and patterning techniques have been used.
Lithographic techniques based upon removing unwanted areas using photoresist as an etch mask have been developed (M. T. Gale and H. W. Lehmann, U.S. Pat. No. 4,534,620, Aug. 13, 1985). For etch processes, however, there are a number of problems which must be overcome. Chemistries which will attack both dielectric materials at comparable rates are needed. Then a masking material must be found which is compatible with that etch. Additional lithographic steps may be required to pattern the masking material. Multi-step etch processes, necessary to produce different filters on the same device, will require an etch stop layer. The process of patterning by etching also does not lend itself readily to changes in dielectric materials or deposition techniques.
Another technique employs a "pseudo-lift-off" process for patterning of brittle, dielectric materials (W. M. Kramer and D. M. Hoffman, J. of Imaging Technology, V. 12, No. 5, October 1986). Conventional positive-working photoresists are lithographically patterned in the usual manner. The filter materials are deposited on top of the resist and onto the substrate through the openings in the resist. Immersion into a solvent removes the resist and unwanted filter areas by a cohesive failure mechanism. The technique relies upon the materials cracking along the edges of the resist pattern and is an inherently unreliable process.
Compared with etch processes, a lift-off process represents a good general purpose technique and offers some advantages in process simplicity. To achieve the maximum process control and resolution, it is desirable that a overhanging or reentry resist sidewall profile be generated. This more traditional technique has been used to pattern filters, however, this requires a resist lift-off process which can be coated thicker than the thickest filter. Typical thicknesses for dielectric stack filters are greater than 1 .mu.m and usually range between 2 and 4 .mu.m. Unfortunately, most lift-off processes which produce reentry sidewall profiles have been developed for metallization purposes where the resist coatings are 1 to 1.5 .mu.m thick.
These types of lift-off processes can be categorized into four groups. Some lift-off systems are based upon combinations of light sources and chemistries which photochemically result in retrograde resist edge profiles after development. Image reversal techniques produce similar profiles. In image reversal, exposed areas are chemically altered to decrease solubility. Unexposed areas are subsequently exposed and developed away. The most widely employed technique is known as the chlorobenzene process. By treating a resist coating with chlorobenzene it is possible to alter the dissolution characteristics of the surface such that the overhang structure is produced during development. Silylation techniques have also been used which modify the etching characteristics of a resist surface. Multi-layer resist technology is an area that has received much investigation. Consequently, many permutations using two or three layers of different materials have been reported. A review article by Frary and Seese, Semiconductor International, pages 72-88 (December 1981), discusses the various approaches that have been explored. Of particular relevance is the discussion of structures comprising two layers of positive-working photoresist. The application of one resist onto another suffers from intermixing of the two similar materials such that the development characteristics gradually change throughout the layers; consequently it is difficult to produce the desired overhang structure. Plasma etch or thermal treatment has been used to alter the surface characteristics of the bottom resist layer to produce a "buffer" layer which prevents intermixing. This process allows the top resist layer to be coated uniformly and maintains the distinction between the two layers. Two resist materials may be chosen such that they either exhibit different dissolution rates in the same developer or else they use mutually exclusive developers. In this case, an overhang structure can be produced. Depending upon the treatment conditions used to form the buffer, it may be necessary to use a two step development process with an intermediate etch step to remove the buffer layer. The use of a double resist lift-off process which provided an improved technique for patterning interference filters has been described in the above referenced Hanrahan patent application. Although this process is effective, the nature of the materials used, however, limits the deposition temperatures to less than 130.degree. C. The physical and optical properties of the final interference filters are optimal when deposited at higher temperatures so that a lift-off process capable of withstanding higher deposition temperatures is desirable.
In terms of ease of manufacturability, the technique used for patterning dichroic filters should be as simple and robust as possible. In other words, the number of processing steps and critical process controls should be minimized. In addition, there should be a wide margin in control factor settings which still result in acceptable product. Etching processes, as indicated previously, require specific etch chemistry and material choices. Suitable masking and etch stop materials must be found that are compatible with the aforementioned etch chemistry. This usually means a pattern transfer process just to produce the appropriate pattern in the etch mask material. Dry etching techniques also have many process controls that have to be monitored and maintained in a manufacturing environment.